A "baffeled box" 4x5x18 inches will hold 1.5 gallons of fuel. It could be made of epoxy soaked lite ply. The sides, top and bottoms could be part of the fuselage structure with the baffles and end plates acting as formers. It could extend from near the firewall to near the trailing edge of the wing with its center near 25% of the wing chord. In this way, the tank wouldn't result in much, if any, weight or frontal area penalty.